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### Question: What causes Wilms' tumor?
### Response: Wilms tumor is the most common form of childhood kidney cancer. The exact cause of this tumor in most children is unknown.
A missing iris of the eye (aniridia) is a birth defect that is sometimes associated with Wilms tumor. Other birth defects linked to this type of kidney cancer include certain urinary tract problems and enlargement of one side of the body, a condition called hemihypertrophy.